Melt the butter and olive oil in a large stockpot over medium heat.
Add the sliced onions and cook, stirring frequently, until they become soft, golden, and caramelized, about 25–30 minutes.
Stir in the beef broth, white grape juice mixture, thyme, salt, and pepper.
Bring to a gentle simmer and cook for another 30 minutes to develop the flavors.
Preheat the broiler.
Ladle the French Onion Soup Brioche Toppers mixture into oven-safe ramekins or soup bowls.
Place one brioche bun half on top of each bowl.
Sprinkle evenly with Gruyère and Parmesan cheese.
Broil for 2–3 minutes until the cheese is bubbly, melted, and lightly golden.
Serve immediately while hot.